1. Field of the Invention
This invention relates generally to a method for quantifying the health of the membranes and electrodes in a fuel cell stack and, more particularly, to a method for estimating a cross-over parasitic current and a shorting resistance of the membranes in a fuel cell stack to determine the health of the fuel cells in the stack.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is renewable and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs require adequate fuel supply and humidification for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack typically includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The MEAs are permeable and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side of the stack, often referred to as nitrogen cross-over. Even though the anode side pressure may be slightly higher than the cathode side pressure, cathode side partial pressures will cause oxygen and nitrogen to permeate through the membrane. The permeated oxygen combusts in the presence of the anode catalyst, but the permeated nitrogen in the anode side of the fuel cell stack dilutes the hydrogen. If the nitrogen concentration increases above a certain percentage, such as 50%, fuel cells in the stack may become starved of hydrogen. If the anode becomes hydrogen starved, the fuel cell stack may fail to produce adequate electrical power and may suffer damage to the electrodes in the fuel cell stack. As the membranes age, they may become thinner, thereby allowing nitrogen to permeate to the anode side at a faster rate.
It is known in the art to provide a bleed valve at the anode exhaust gas output of the fuel cell stack to remove nitrogen from the anode side of the stack. It is also known in the art to estimate the molar fraction of nitrogen in the anode side using a model to determine when to perform the bleed of the anode side or anode sub-system. However, the model estimation may contain errors, particularly as degradation of the components of the fuel cell system, such as the membranes, occurs over time. If the anode nitrogen molar fraction estimation is significantly higher than the actual nitrogen molar fraction, the fuel cell system will vent more anode gas than is necessary, i.e., waste fuel. If the anode nitrogen molar fraction estimation is significantly lower than the actual nitrogen molar fraction, the system will not vent enough anode gas and may starve the fuel cells of reactants, which may damage the electrodes in the fuel cell stack.
Depending on power requirements of a fuel cell system, the voltage of the fuel cell stack changes. This is known as voltage cycling of the stack. Voltage cycling causes the catalyst particles to change, for example, the catalyst particles may aggregate, thereby reducing the surface area on which the electrochemical reaction may take place. This causes inefficiencies and diminishes the durability of the fuel cell. Additionally, aggregation of catalyst particles may cause the catalyst support to collapse. Corrosion of the catalyst layers can also occur, which also reduces the durability of the fuel cell.
There is a need in the art to determine the health of the electrodes and the membranes in a fuel cell stack throughout the lifetime of the membranes in the stack in a way that may be performed on a fuel cell stack in a vehicle without requiring the vehicle to come in for servicing and without requiring burdensome testing conditions that may affect the normal operation of the vehicle. The ability to quantify electrode and membrane health in a fuel cell vehicle will provide a multitude of possibilities to optimize vehicle efficiency and power based on driving demands.